Method for producing fillers having improved bulk material stability and pourability

ABSTRACT

Disclosed is a method for producing surface-modified fillers by spray-drying an aqueous suspension of a filler that consists of fine particles, whereby 0.1 to 50 wt. % (calculated as dry substance), preferably 0.3 up to less than 1 wt %, of an aqueous dispersion of a thermoplastic polymer or copolymer or a caoutchouc are added. Said fillers have improved bulk material stability and improved pourability. The inventive fillers are particularly suitable as flame-retardant fillers for synthetic materials or rubber, in a cable insulation for instance.

[0001] The invention relates to a process for the preparation offine-particle fillers, in particular those with flame-retardant actionsuch as aluminium hydroxide or magnesium hydroxide, with improvedfree-flowability and improved bulk density behaviour, in particularafter pneumatic conveyance through pipes, as well as polymer-coatedfillers obtainable according to this process and their use asflame-retardant fillers in plastic or rubber compounds.

[0002] Fine-particle precipitated or also ground fillers often display apoor free-flowability. This impairs the flowability of the products,e.g. in compounding units or in those conveyance processes in which agood flow of the product is an essential criterion (e.g. silo discharge,flow in the internal mixer, etc.).

[0003] Although an improved free-flowability is achieved by addingdispersing agents customary in the trade, the products thus obtainedgenerally have an electrical conductivity that is much too great forcable insulation applications.

[0004] A further disadvantage of these products is that after aconveyance process (e.g. through freefall or by means of air through apipe) the bulk density sinks to a very low level. As a result, thecompounding behaviour worsens at the same time in all current mixingunits.

[0005] A further aspect is that the trend on the market is towards batchdelivery. This means that even after a transport process, for examplefrom the manufacturer's silo to the silo vehicle and from there into thecustomer's silo with subsequent conveyance into the service hopper, ahigh bulk density combined with good free-flowability is desirable.

[0006] Further important limiting conditions are that the properties ofthe plastic or rubber compounds prepared with these products must beunimpaired or only slightly impaired. These include in particular:

[0007] the rheological properties of the compounds

[0008] the mechanical properties of the compounds

[0009] the flame protection, where flame-retardant fillers are involved

[0010] the electrical properties of the compounds, in particular afterageing in water

[0011] silane-coupling to the filler, here primarily amino and vinylsilanes

[0012] the freedom from hard filler agglomerates

[0013] With regard to freedom from agglomerates, although agglomeratesmay be present in the filler, they must dissolve completely in theplastic during the compounding process.

[0014] In addition, the additive must not lead to a reduction in thethroughput during filler production, for example during the spray-drying(e.g. because of a reduction in the solids content in the slurry or anincrease in the viscosity of the slurry).

[0015] Furthermore an increased moisture affinity of the coated productis to be ruled out because this leads, e.g. in electrical insulationmaterials, to blister formation and thus among other things to a reduceddielectric strength.

[0016] The object of the invention was therefore to preparefine-particle flame-retardant fillers which have a goodfree-flowability, display a high bulk density even after pneumaticconveyance, can be worked well into plastic compounds and the typicalproperties of which, in particular when used in cable compounds, are notdisadvantageously influenced.

[0017] According to the invention, this object is achieved by theprocess according to patent claim 1.

[0018] It was found that by adding 0.1 to 50 wt.-%, preferably 0.3 toless than 1 wt. %, suitable additives to an aqueous filler suspensionand then spray-drying, the desired properties can be achieved. Thequantities given relate in each case to the dry substance, i.e. thefiller on the one hand and the polymers or copolymers on the other. Bysuitable additives are meant according to the invention aqueous polymeror copolymer emulsions, at least one thermoplastic polymer or copolymeror a rubber (latex) or a mixture of several of the named polymericsubstances being present.

[0019] It is surprising in particular that quantities of less than 1.0wt.-% of the named polymers are already sufficient to achieve the effectaccording to the invention.

[0020] The polymer dispersions usually contain an emulsifier for theaqueous phase, anionic, cationic or also neutral emulsifiers being ableto be used.

[0021] Natural or synthetic substances such as for example calciumcarbonate, dolomite, barium sulphate, talc, china clay, mica,wollastonite, silicon dioxide (silica, silicic acid), aluminium oxide,magnesium oxide, aluminium hydroxide and magnesium hydroxide can be usedas fillers.

[0022] Usable as thermoplastic polymers are for example styrenecopolymers such as e.g. styrene/acrylonitrile copolymers (SAN) or SANmodifications such as e.g. acrylonitrile/polybutadiene/styrene graftpolymers, or graft copolymers based on methyl methacrylate orpolyacrylates such as e.g. acrylonitrile copolymers or graft copolymersof polymethyl methacrylate with polybutadiene. Copolymers of vinylchloride and vinylidene chloride/acrylonitrile are also suitable.

[0023] Polyvinyl alcohol (PVOH) and also polyvinyl acetate (PVA) arepreferred.

[0024] Also preferred are polymerized esters or copolymerized esters ofacrylic acid with low alcohols, in particular those with C₁₋₆ alcoholssuch as methyl acrylate, ethyl acrylate or butyl acrylate. These alsoinclude for example ethylene/ethyl acrylate copolymers (EEA),ethylene/methyl acrylate copolymers (EMA) and ethylene/butyl acrylatecopolymers (EBA).

[0025] Polyethylene or copolymers of ethylene are also preferably used.Examples are: ethylene/vinyl chloride graft copolymers such asethylene/vinyl chloride/vinyl acetate (EVCVA) and ethylene/acrylic acidcopolymers (EAA).

[0026] Ethylene/vinyl acetate copolymers (EVA) and ethylene/vinylalcohol copolymers (EVOH) are particularly preferred.

[0027] In a likewise preferred version, the polymer dispersion containsat least one self-cross-linking polymer or copolymer.

[0028] The polymer dispersions used according to the invention can beadded for example in metered doses together with a dispersing agentrequired for the formation of the filler suspension. To prepare thefiller suspension, e.g. the ground dry filler is liquefied with waterand the dispersing agent, or in the case of synthetic (precipitated)fillers, the moist filler obtained after precipitation and filtration isused.

[0029] The spray-drying is advantageously carried out [with] asuspension the filler content of which is between 10 and 90 wt.-%,preferably between 40 and 65 wt.-%. Spray dryers customary in the tradecan be used as can be obtained for example from Niro. Air is preferablyused as desiccant gas, the quantity and entry temperature of which isadvantageously such that an exit temperature of 100-150° C. results.

[0030] The process according to the invention is particularly preferablyused for the preparation of fillers based on aluminium and/or magnesiumhydroxides. Both single hydroxides such as Al(OH)₃, AlOOH and Mg(OH)₂,and physical mixtures of these hydroxides or mixed hydroxides such ashydrotalcite or similar compounds can also be used.

[0031] A further particularly preferred filler is calcium carbonate.

[0032] A product with a primary particle size (given as d₅₀ value) ofnot more than 15 μm, preferably not more than 5 μm, is advantageouslyused as filler. The free-flowable product, improved in respect of bulkdensity behaviour, i.e. conveyance-stable, which can be prepared fromsame according to the invention has an average agglomerate size of 1 to500 μm, preferably 20 to 200 μm, and, according to the quantity ofpolymer dispersion used, a coating with 0.1 to 50 wt.-%, preferably 0.3to less than 1 wt.-% of a polymer and/or copolymer.

[0033] The free-flowable fillers according to the invention arepreferably used in a quantity of 5 to 90 wt.-% (relative to the overallweight of the compound) as flame-retardant fillers in plastic or rubbercompounds, in particular in cable mixtures.

[0034] The following examples with aluminium hydroxide of differentprimary particle sizes illustrate the performance of the inventionwithout being regarded as limiting it in any way.

[0035] The aluminium hydroxide used in the examples as startingmaterials had the properties summarized in the following Table 1: TABLE1 Type Particle size (d₅₀) [μm] Spec. surface (BET) [m²/g] OL-104/LE1.3-2.3 3-5 OL-107/LE 0.9-1.5 6-8 SF4 ESD 1.96 4.5

[0036] Types OL-104/LE and OL-107/LE are products of AlusuisseMartinswerk Gmbh, Bergheim/Erft, Germany. SF4 ESD is an Alcan product.All types were used as product customary in the trade.

[0037] The mixing ratios of the compounds are given in the customary phrunit (1 phr=1 part by weight per 100 parts by weight polymer).

EXAMPLE 1 Influence on Electrical Conductivity in Water and Viscosity ofthe Slurry

[0038] The influence of the additives according to the invention onelectrical conductivity and viscosity were examined (Brookfieldviscometer, 100 rpm, spindle 3). To this end, 0.5 wt.-% or 1.0 wt.-%polymer active ingredient, relative to aluminium hydroxide, were addedto an aqueous slurry with 55 wt. % aluminium hydroxide (type OL-104/LE)and the viscosity then measured. For problem-free further processing,the viscosity should be below 200 mPa·s as far as possible.

[0039] Electrical conductivity was measured in the same suspension whichwas diluted for this purpose to 10% solids content by means of distilledwater. To obtain good electrical properties of the products, theconductivity should be below 150 μS/cm as far as possible. The resultsare summarized in the following Table 2. TABLE 2 Active ingredientConductivity Viscosity Additive Polymer type [%] [μS/cm] [mPa · s] 1Polyvinyl alcohol 0.5 51 58 PAF 60¹⁾ 1.0 54 58 2 Polyvinyl alcohol 0.551 58 PAF 2¹⁾ 1.0 57 56 3 Ethylene/vinyl 0.5 81 47 acetate 1.0 110 47copolymer Vinamul ® 3242²⁾ 4 Self-cross-linking 0.5 93 57 ethylene/vinyl1.0 124 55 chloride/vinyl acetate copolymer Vinamul ® 3479²⁾ 5Ethylene/vinyl 0.5 75 40 chloride/vinyl 1.0 75 22 acetate/acrylatecopolymer Vinamul ® 3650²⁾ 6 Polyvinyl acetate 0.5 76 45 Vinamul ®9300²⁾ 1.0 86 53 7 Self-cross-linking 0.5 71 44 polyacrylate 1.0 76 41Vinacryl ® 4345²⁾

EXAMPLE 2 Influence on Conveyance Behaviour and Free-Flowability

[0040] Selected additives were sprayed with a suspension of 55 wt.-%aluminium hydroxide (OL-104/LE) and an active ingredient content of 0.5wt.-%, relative to the aluminium hydroxide, on a semi-scale plant spraydryer (Niro Atomizer, “Production Minor” type). A dispersing agent (e.g.acetic acid) was used and the additives stirred in as aqueousdispersion. The throughput of the spray dryer was approx. 10 kg/hsolids, the air-supply temperature approx. 500° C. and the waste-airtemperature 120-130° C. Before and after a conveyance process, the bulkdensity, on a litre scale (“litre weight”), and the free-flowability,quantified by the flow time, of the spray-dried product were measured bymeans of suction air (25 m pipe, Ø=40 mm, with 9 bends each of 90°). Tomeasure the litre weight a cylindrical measuring vessel with a diameterof 110 mm and a volume of 1 l was filled to the brim and the net weightmeasured. The flow time was measured as the throughflow time of 100 gfiller through a polished frustrum-shaped brass funnel with a height of115 mm, an upper diameter of 145 mm and an outflow diameter of 16 mm.The funnel is vibrated by a mechanical oscillator of defined frequencyand amplitude. An AS 200 type control screening machine from Retschserved for the following tests. The frequency was fixed in advance bythe design, the amplitude was 1.5 mm. The results are summarized in thefollowing Table 3. In each case the litre weight is given and the flowtime before and after the conveyance compared with the untreatedproducts OL-104/LE and SF4 ESD. The types and manufacturers of theadditives are the same as in Table 2. TABLE 3 Litre weight Litre weightFlow time Flow time before after before after conveyance conveyanceconveyance conveyance Additive Polymer type [g/l] [g/l] [s] [s] — — 570362 11 20  —* — 515 327 8 14 1 Polyvinyl 624 564 11 9 alcohol 2Polyvinyl 640 515 9 10 alcohol 3 Ethylene/vinyl 543 433 9 13 acetatecopolymer 4 Self-cross- 650 468 14 10 linking ethylene/vinylchloride/vinyl acetate copolymer 5 Ethylene/Vinyl 500 419 7 9chloride/vinyl acetate/acrylate copolymer 6 Polyvinyl 507 416 8 11acetate 7 Self-cross- 613 492 13 8 linking acrylate

[0041] Through the conveyance process, the litre weight of the OL-104/LEdecreases from 570 g/l to 362 g/l and that of the SF4 ESD from 515 g/lto 327 g/l, i.e. in each case by approximately 36%.

[0042] The increase, i.e. the worsening of the flow time of OL-104/LEfrom 11 s to 20 s, is also clear. Although the product SF4 ESD hasshorter flow times in absolute terms, the relative increase afterconveyance is approximately the same. On the other hand, the productsaccording to the invention simultaneously display an improved flowbehaviour and a clearly smaller decrease in the litre weight through theconveyance. The smallest decrease (approx. −10%) was obtained withadditive 1, the largest (approx. −28%) with additive 4, in each casehowever it was clearly smaller than with the reference products notaccording to the invention. In the case of the products according to theinvention, a decrease in the flow time was even observed after theconveyance process with additives 1, 4 and 7. This flow improvementthrough the conveyance process appears paradoxical, but can be explainedby a reduction in the “adhesive effect” by the polymer duringconveyance.

EXAMPLE 3 Influence on the Conveyance Behaviour and Free-Flowability

[0043] The procedure was as described in Example 2, however instead ofaluminium hydroxide OL-104/LE, the finer-particle grade OL-107/LE wasused as filler starting material. In addition to the two polyvinylalcohol types from Examples 1 and 2, four self-cross-linkingpolyacrylates from Vinamul were used as additives. The results aresummarized in the following Table 4. TABLE 4 Litre weight Litre weightFlow time Flow time before after before after conveyance conveyanceconveyance conveyance Additive Polymer type [g/l] [g/l] [s] [s] — — 389282 13 23 1 Polyvinyl 489 473 9 10 alcohol PAF 60 2 Polyvinyl 498 460 912 alcohol PAF 2 7 Self-cross- 445 426 10 9 linking acrylate Vinacryl ®4345 8 Self-cross- 495 408 15 9 linking acrylate Vinacryl ® 4343 9Self-cross- 460 443 9 9 linking acrylate Vinacryl ® 4344 10 Self-cross-334 306 8 13 linking acrylate Vinacryl ® 4373

EXAMPLE 4 Influence on the Physical Properties of Compounds Without andWith Silane

[0044] To examine the influence of the coating according to theinvention on the rheological, mechanical and electrical properties, theflame protection and the silane coupling, the 4 additives no. 1, no. 2,no. 4 and no. 7 were selected for further tests. Melt Flow Index (MFI)in accordance with DIN-ISO 1133, tensile strength σ_(B) and elongationat break ε_(B) in accordance with DIN 53504 and EN ISO 527, specificresistance p after storage in water for 7 d at 70° C. or 28 d at 50° C.in accordance with DIN 53482/VDE 0303 Part 3 and Limiting Oxygen Index(LOI) in accordance with ASTM D2863 were measured. Tables 5 and 6 showthe results compared with the uncoated OL-194/LE and SF4 ESD in an EVAcompound containing 19% vinyl acetate (polymer=100 phr), 150 phr fillerquantity, 1.5 phr (3-aminopropyl)triethoxy silane, which was addedduring the compounding process on a rolling mill, and 0.75 phr Irganox®1010. TABLE 5 (without silane) MFI ρ ρ LOI (190° C./10 σ_(B) ε_(B) 7d/70° C. 28 d/50° C. (50 × 2 mm²) Additive kg [g/10 min] [Mpa] [%] [Ω ·cm] [Ω · cm] [% O₂] — 1.24 8.8 119 2.9 · 10⁸ 2.2 · 10⁸ 38  —*) 1.41 9.2114 — 5.2 · 10⁷ 38 1 1.72 7.8 140 5.3 · 10⁹ 1.9 · 10⁸ 37.6 2 1.92 8.5135 4.0 · 10⁹ 9.4 · 10⁷ 37.8 4 1.52 8.7 147 3.5 · 10⁹ 9.0 · 10⁸ 37.4 71.52 9.3 170 4.6 · 10⁸ 1.3 · 10⁹ 37.4

[0045] TABLE 6 (with silane) MFI ρ (190° C./10 kg) σ_(B) ε_(B) 28 d/50°C. Additive [g/10 min] [Mpa] [%] [Ω · cm] — 0.9 11.6 205 1.9 · 10¹⁴  —*⁾1.1 11.6 197 2.9 · 10¹⁴ 1 1.4 11.6 197 1.8 · 10¹³ 2 1.5 11.6 202 1.8 ·10¹⁴ 4 1.3 11.4 208 1.5 · 10¹³ 7 1.2 11.5 210 3.5 · 10¹⁴

[0046] Table 5 shows that the melt flow index (MFI) is slightly improvedby the process according to the invention. Without aminosilane, thetensile strength is not significantly changed with the exception ofadditive no. 1. The elongation at break on the other hand issignificantly improved. The electrical properties after 7 d/70° C. or 28d/50° C. are the same as or even better than for untreated OL-104/LE andalways better than for the comparison product SF4 ESD.

[0047] Table 6 also shows that the MFI is slightly improved comparedwith OL-104/LE and SF4 ESD. The observed mechanical properties provethat the silane coupling is apparently not impaired. Even after ageingin water, very high specific resistance values of 10¹³ to 10¹⁴ Ω·cm areobtained.

EXAMPLE 5 Influence on the Mechanical Compound Properties in aCross-Linkable EVA Formulation with Vinyl Silane

[0048] Compounds were prepared according to the following formulation:

[0049] 100 phr EVA (vinyl acetate content 19%)

[0050] 150 phr aluminium hydroxide

[0051] 1.5 phr vinyltrimethoxysilane

[0052] 2.5 phr Perkadox® 14/40 peroxide

[0053] 0.75 phr Irganox® 1010.

[0054] Aluminium hydroxide, or OL-107/LE, untreated or treated accordingto the invention as per Example 2, was used as aluminium hydroxide. Thevinyl silane was added during the compounding on the rolling mill. Thecompounds were cross-linked for 20 min at 175° C. Tensile strength σ_(B)and elongation at break ε_(B) were then measured. The results aresummarized in the following Table 7. TABLE 7 Additive σ_(B) [Mpa] ε_(B)[%] — 18.3 193 1 18.1 163 2 18.5 178 7 19.4 198 8 17.9 207

[0055] According to Table 7, the measured values show only small changescompared with the uncoated product, and these either lie in themeasurement value spread range or can be optimized by adjustment of thesilane/peroxide quantity.

1. Process for the preparation of surface modified fillers with improvedbulk density stability, improved free-flowability and an averageagglomerate size from 1 to 500 μm, characterized in that an aqueoussuspension of filler with a primary particle size in the d₅₀-value ofnot more than 15 μm with addition of 0.1 to 50 wt.-% (calculated as drysubstance) of an aqueous polymer dispersion is spray-dried, whichaqueous polymer dispersion consists of at least one thermoplasticpolymer or copolymer and/or one rubber and optionally at least oneself-crosslinking polymer or copolymer, dispersing agent and water. 2.Process according to claim 1 characterized in that the polymerdispersion is added in a quantity (calculated as dry substance) of 0.3to less than 1.0 wt.-%.
 3. Process according to claim 1 or 2,characterized in that the polymer dispersion contains polyvinyl alcohol.4. Process according to any of claims 1 to 3, characterized in that thepolymer dispersion contains polyvinyl acetate.
 5. Process according toany of claims 1 to 4, characterized in that the polymer dispersioncontains at least one polymerized or copolymerized ester of acrylic acidwith low alcohols, in particular C₁₋₆ alcohols, such as butyl acrylate.6. Process according to any of claims 1 to 5, characterized in that thepolymer dispersion contains polyethylene or a copolymer of ethylene. 7.Process according to claim 6, characterized in that the polymerdispersion contains an ethylene/vinyl acetate copolymer.
 8. Processaccording to claim 6 or 7, characterized in that the polymer dispersioncontain ethylene/vinyl alcohol copolymer.
 9. Process according to any ofclaims 1 to 8, characterized in that the polymer dispersion contains atleast one self-crosslinking polymer or copolymer.
 10. Process accordingto any of claims 1 to 9, characterized in that the spray-drying iscarried out with a filler content of 10 to 90 wt.-%, preferably 40 to 65wt.-%.
 11. Process according to any of claims 1 to 10, characterized inthat one or more hydroxides of aluminium and/or magnesium are used asfiller.
 12. Process according to any of claims 1 to 10, characterized inthat calcium carbonate is used as filler.
 13. Free-flowable fillers witha primary particle size in the d₅₀ value of not more than 15 μm,preferably not more than 5 μm, an average agglomerate size of 1 to 500μm, preferably 20 to 200 μm, and a coating with 0.1 to 50 wt.-% ofpolymer or copolymer, which consists at least of one thermoplasticpolymer or copolymer and/or one rubber, and optionally at least oneself-crosslinking polymer or copolymer, dispersing agent and water,obtainable by the process according to claims 1 to
 12. 14. Free-flowablefillers according to claim 13, characterized in that they carry acoating of 0.3 to less than 1.0 wt.-% of polymer or copolymer.
 15. Useof free-flowable fillers according to claim 13 or 14 as flame retardandfillers in a quantity of 5 to 90 wt. % in plastic or rubber compounds,in particular in mixtures for cable insulations.